The uniformity requirements for flat surfaces and curved surfaces are related to their usage. Nothing is perfectly flat or perfectly cylindrical; however, many machine parts are required to be flat to within an accuracy of 0.00001 inch. Rollers used in the manufacture of thin films and foils have similar accuracy requirements. The flatness or curvature of mirrors must be checked to assure proper manufacture. NASA recently learned a sad lesson on the consequences of not properly testing the curvature of the Hubbell telescope mirror.
Instruments available for the measurement of flatness are typically autocollimators or are based on the interferometer principal. The autocollimator projects a light beam that is reflected onto a detector from a reflector unit which is moved across the test surface. The autocollimator measures the direction of the reflection which represents the minute variations in the slope across the surface. A profile is determined from the slope values. Interferometers interfere a beam of coherent light with a reflected beam to measure slopes in terms of the wavelength of the light. These devices are typically complicated to use.
The roundness of cylinders up to about two feet are typically checked with micrometer type devices. What is needed is an easy to operate instrument which can measure flatness, straightness and uniformity of curvature with no upper size limits.